IDIOMS

Well, I don't have any favourite idiom. In fact, when speaking they don't come into my mind. I think using idioms in your daily speech is a further step for an advanced learner because that means that not only you master the grammar and the lexis but also cultural aspects.
I couldn't think of my favourite idiom, then I checked a link in our blog (goenglish.com) and found a list. I picked out the following:

• to be as mad as a hatter. Do you remember Alice in Wonderland? There was a rabbit who celebrated no birthday days. Well, he was the Mad Hatter. It turns out that a hatter was a professional in England who mended and made hats using a glue which affected his brain and led him to act strange.
• on the hook. That means to be in a difficult situation. For example: Oh, you've broken mum's ashtray. You are on the hook. The opposite is to be off the hook.